1. Field of the Invention
The present invention concerns a two-way microwave communications circuit between a central station or fixed beacon, fitted out with a signal transmission and reception system, and a terminal station or independently powered mobile device. More precisely, the invention relates to a circuit known as an "active antenna" which, in the portable badge type mobile device, maintains a watch for interrogation signals and, having detected them, activates the device so that, in turn, this mobile device transmits encoded signals with greater power in response to the interrogation.
2. Description of the Prior Art
This type of data exchange can be applied, for example, to the remote management of toll-charges, remote cash dispensing or to the identification or localization of moving bodies. The mobile device takes the form of a chip card, carried by a person, or the form of a moving object. Supplied by small "button" batteries, it has to be extremely economical in its use of power.
The block diagram of this type of data exchange is known per se and is recalled in FIG. 1. A beacon 1 exchanges microwave signals with a portable badge 2 that includes an active antenna 3 and control circuits 4, these elements 3 and 4 being supplied with energy by a supply 5, essentially formed by one or more batteries.
The active antenna 3 itself comprises:
one or two transmission/reception antenna 6-7;
a microwave modem circuit 8 to modulate and demodulate the information transmitted in the form of logic pulses;
a low frequency/microwave interface circuit 9, to enable the link with the control circuits 4 which include, for example, a microprocessor and memories.
The modulation used in this application is of the OOK type, i.e. it is an all or nothing type amplitude modulation of the microwave carrier. In an interrogation mode, the beacon sends out a modulated wave in pulses of varying lengths. In reception mode, the beacon sends out a pure wave and the portable badge sends out a modulated wave.
As a consequence, the badge should be capable of working in two states:
in reception, under very low energy consumption (some microamperes) for this is its almost permanent state, it should be capable of receiving and demodulating an interrogation signal sent out by the fixed beacon;
in transmission, after having identified an interrogator, it should be capable of sending out a signal having a specific modulation with sufficient . power (some milliamperes).
It is therefore necessary to switch over the portable badge, to make it go from one state to another state, in the simplest possible way so that consumption in the watching state is very low.